1. Field of the Invention
The present disclosure relates to sensors, and, more particularly, to CMOS image sensors and a method of manufacturing the same.
2. Discussion of the Related Art
Generally, image sensors convert an optical image into an electrical signal. Recently, as the information communication industry and digital electronic devices have developed, image sensors are being widely used in diverse fields such as digital cameras, camcorders, cellular phones, personal communication systems, game devices, guard cameras, medical micro cameras, etc.
Typically, when the degree of integration of a pixel in an image sensor is increased to improve the resolution of the image sensor, the sensitivity of the image sensor is decreased due to the small volume of the photoelectric transforming element, for example, a photodiode of a unit pixel.
As semiconductor devices become highly integrated, the area of a unit cell has decreased. However, the requirement for highly integrating the semiconductor device may produce a short distance between adjacent pixels, so that crosstalk is frequently generated.
A conventional CMOS image sensor includes a unit cell having a photodiode region and a peripheral circuit region on a semiconductor substrate. Optical crosstalk and/or electrical crosstalk is generated in the conventional CMOS image sensor cell. The optical crosstalk is generated by the refraction of light which is incident to the cell through a micro lens on insulating interlayers having different refractivities or a layer having uneven surface. The electrical crosstalk is generated by electrons thermally created outside of a depletion region in the semiconductor substrate. The optical crosstalk and/or the electrical crosstalk can cause a low resolution and a distorted image of the conventional image sensor.
When the conventional image sensor is produced, the gap between the pixels of the image sensor can be quite narrow and result in the image sensor malfunctioning due to crosstalk.
Recently, with the development of the information communication industry and of digital electronic devices in general, highly improved image sensors are widely used in diverse fields such as digital cameras, camcorders, cellular phones, personal communication systems, game devices, guard cameras, medical micro cameras, etc. Further, since semiconductor devices have become highly integrated, the area of a unit cell in the semiconductor device has become greatly reduced, so that the width of patterns and the gap between patterns has narrowed. In contrast, the semiconductor devices still require good electrical characteristics and low power consumption in spite of the small area of the unit cell.
A conventional image sensor cell includes an active pixel sensor (APS) array region which includes a photodiode region and a peripheral circuit region.
A CMOS image sensor includes a peripheral circuit and a photodiode having a CMOS cell structure by forming an epitaxial layer on a P type substrate. Light which is incident to the cell through a micro lens is refracted from insulating interlayers having different refractivities or a layer having uneven surface to generate an optical crosstalk. Further, electrons which are thermally created outside of the depletion region in the semiconductor substrate diffuse in various directions because an electric field does not exist. The electrons can accumulate in the pixel and generate electrical crosstalk. The optical crosstalk and/or the electrical crosstalk cause low resolution and a distorted image of the conventional image sensor.
Because the conventional image sensor includes a P type substrate, although having good sensitivity, the conventional image sensor has inferior characteristics with respect to dark current (i.e, the relatively small electric current that flows through a photosensitive device when no photons are entering the device) and crosstalk. In contrast, when an N type substrate is used, a voltage of the N type substrate is controlled through an operational voltage terminal. Thus, the thermal electron hole pair which is generated outside of the depletion region in the semiconductor substrate does not diffuse in various directions. The thermal electron is drained through the operational voltage terminal. The hole is drained through a ground terminal.
When the P type substrate is used, there is not an exit passageway through which the thermal electrons can pass and the thermal electrons accumulate in the photodiode.